Label Spaces, LDP Sessions, and Hello Adjacencies

THE LABEL DISTRIBUTION PROTOCOL LDP 155 A D Reserved PVLim Receiver LDP identifier Max PDU length Common sess parms Length KeepAlive time Protocol version 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Figure 7.6 Common session parameters TLV. • D : Enables loop detection. • PVLim Path vector limit : Gives the maximum number of LSRs recorded in the path vector used for loop detection. • Max PDU length : Default value of the maximum allowable length is 4096 bytes. • Receiver LDP identifier : Identifies the receiver’s label space. The optional session parameters field can be used to provide for ATM and frame relay session parameters. KeepAlive message An LSR sends keepAlive messages as part of the a mechanism that monitors the integrity of an LDP session. The format of the keepAlive message is shown in Figure 7.4, with the U bit set to 0, and the message type set to keepALive 0x0201. No mandatory or optional parameters are provided. Address and address withdraw messages Before sending a label mapping and a label request messages, an LSR advertises its interface addresses using the address messages. Previously advertised addresses can be withdrawn using the address withdraw message. Label mapping message An LSR uses the message to advertise a mapping i.e., a binding of a label to a FEC to its LDP peers. The format of the label mapping message has the same structure as the one shown in Figure 7.4, with the U bit set to 0 and the message type set to label mapping 0x0400. The mandatory parameters field consists of a FEC TLV and a label TLV. In LDP a FEC element could be either a prefix of an IP address or it could be the full IP address of a destination host. The FEC TLV is shown in Figure 7.7. LDP permits a FEC to be specified by a set of FEC elements, with each FEC element identifying a set of packets that can be mapped to the corresponding LSP. This can be useful, for instance, when an LSP is shared by multiple FEC destinations all sharing the same path. The label TLV gives the label associated with the FEC given in the FEC TLV. This label could be a 20-bit label value, or a VPIVCI value in the case of ATM, or a DLCI 156 LABEL DISTRIBUTION PROTOCOLS 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 1 0 0 FEC element 1 FEC element n Length FEC 0x0100 … Figure 7.7 The FEC TLV. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 1 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 1 Res V VPI VCI ATM label 0x0201 Length Generic label 0x0200 Label Length Figure 7.8 The generic label and ATM label TLVs. value in the case of frame relay. The generic label and ATM label TLVs are shown in Figure 7.8. The 2-bit V field in the ATM label TLV, referred to as the V-bits, is used as follows. If the V-bits is 00, then both the VPI and the VCI fields are significant, If the V-bits is 10, then only the VCI is significant. Label request message An LSR sends a label request message to an LPD peer to request a mapping to particular FEC. The label request message has the format shown in Figure 7.4, with the U bit set to 0, and the message type set to label request 0x0401. The mandatory parameters field contains the FEC TLV shown in Figure 7.7. An LSR can transmit a label request message under the following conditions: • The LSR recognizes a new FEC via its forwarding routing table; the next hop is an LDP peer; and the LSR does not already have a mapping from the next hop for the given FEC. • The next hop to the FEC changes, and the LSR does not already have a mapping from the next hop for the given FEC. • The LSR receives a label request for a FEC from an upstream LDP peer; the FEC next hop is an LDP peer; and the LSR does not already have a mapping from the next hop. THE CONSTRAINED-BASED ROUTING LABEL DISTRIBUTION PROTOCOL 157 Label abort, label withdraw, and label release messages An LSR A can send a label abort message to an LDP peer LSR B to abort an outstanding label request message. This might happen, for instance, if LSR A’s next hop for the FEC has changed from LSR B to a different LSR. An LSR A uses a label withdraw message to signal to an LDP peer LSR B that it cannot continue using a specific FEC-label mapping that LSR A had previously advertised. An LSR A sends a label release message to an LDP peer LSR B to signal to LSR B that LSR A no longer needs a specific FEC-label mapping that was previously requested of andor advertised by the peer.

7.2 THE CONSTRAINED-BASED ROUTING LABEL DISTRIBUTION

PROTOCOL CR-LDP CR-LDP is a label distribution protocol based on LDP. As described above, LDP can be used to set up an LSP associated with a particular FEC. CR-LDP is used to set up a unidirectional point-to-point explicitly routed LSP, referred to as the constrained-based routed label switched path CR-LSP. An LSP is set up as a result of the routing information in an IP network using the shortest path algorithm. A CR-LSP is calculated at the source LSR based on criteria not limited to routing information, such as explicit routing and QoS-based routing. The route then signaled to the other nodes along the path which obey the source’s routing instructions. This routing technique, referred to as source routing, is also used in ATM. A CR-LSP in MPLS is analogous to a connection in ATM, only it is unidirectional. The ATM signaling procedures will automatically set up a bidirectional connection between two ATM hosts, where each direction of the connection can be associated with different traffic and QoS parameters. A bidirectional CR-LSP between LSRs 1 and 2 can only be created by setting up one CR-LSP from LSR 1 to LSR 2 and a separate one from LSR 2 to LSR 1. As in the case of an LSP, a CR-LSP has an ingress and an egress LSR. CR-LSPs can be used in a variety of ways. For instance, they can be used in an IP network to do load balancing. That is, the traffic among its links can be evenly distributed by forcing some of the traffic over CR-LSPs, which pass through lesser-utilized links. CR-LSPs can also be used to create tunnels in MPLS, and introduce routes based on a QoS criterion, such as minimization of the total end-to-end delay, and maximization of throughput. For example, let us consider the MPLS network in Figure 7.9, and let us B A E C F D G Figure 7.9 An example of a CR-LSP.